1. Field of the Invention
The present invention relates to a signal reproducing apparatus including a magneto-optical disk drive for recording and/or reproducing information with laser light by using a magneto-optical effect and, more particularly, to a signal reproducing apparatus for reproducing information recorded with a pit length smaller than an optical diffraction limit.
2. Related Background Art
Recently, research is being extensively done to increase the storage capacity of an optical disk as a storage medium, CD-ROMs and DVDs have become commercially available, and writing once and rewritable optical disks are gradually being put into practical use. Also, various recording and reproducing methods for these recording media have been proposed. Examples are laser pulse magnetic field modulation, a magnetic super resolution technique, an optical super resolution technique, and a phase change optical disk method.
Of means for improving the linear recording density of an optical disk, Japanese Patent Application Laid-Open No. 6-290496 has disclosed a magneto-optical recording medium, a reproducing method, and a reproducing apparatus as a recording medium in which a signal is recorded with a pit length smaller than an optical diffraction limit and means for reproducing the signal. This reproducing method uses a magneto-optical recording medium formed by sequentially stacking at least first, second, and third magnetic layers. The first magnetic layer is made of a perpendicular magnetization film having a smaller magnetic wall coercivity and a larger magnetic wall mobility than those of the third magnetic layer at a temperature near the ambient temperature. The second magnetic layer has a Curie temperature lower than those of the first and third magnetic layers. The third magnetic layer is a perpendicular magnetization film. A light beam is irradiated from the side of the first magnetic layer while being moved relative to the medium, thereby forming a temperature profile having a gradient with respect to the moving direction of the spot of the light beam on the medium. This temperature profile is made to have a region at a temperature higher than at least the Curie temperature of the second magnetic layer. Consequently, magnetic walls formed in the first magnetic layer are moved, and a change in the plane of polarization of the reflected light of the light beam is detected. In this manner, recorded information is reproduced.
Another reproducing method using a medium including a fourth layer in addition to the above three layers is also proposed. This fourth layer is formed between the first and second magnetic layers and has a Curie temperature higher than that of the second magnetic layer and lower than that of the first magnetic layer. The fourth layer is made of a perpendicular magnetization film having a smaller magnetic wall coercivity than that of the third magnetic layer at a temperature higher than at least the Curie temperature of the second magnetic layer. A light beam is irradiated from the side of the first magnetic layer while being moved relative to the medium, thereby forming a temperature profile having a gradient with respect to the moving direction of the spot of the light beam on the medium. This temperature profile is made to have a region at a temperature higher than at least the Curie temperature of the second magnetic layer and close to the Curie temperature of the fourth magnetic layer. Consequently, magnetic walls formed in the first and fourth magnetic layers are moved and a change in the plane of polarization of the reflected light of the light beam is detected. In this manner, recorded information is reproduced. This recording medium and the basic operating principle of the reproducing method using the medium are described in Japanese Patent Application Laid-Open No. 6-290496.
In the above reproducing method, however, it is necessary to raise the temperature from the front of the light beam spot to form a temperature profile having a peak after the spot. That is, as described in the above reference, a means for forming such a temperature profile can be realized by using a heating light source (laser) having a wavelength different from the wavelength of a reproducing beam and irradiating this heating light beam having a larger beam diameter than that of the reproducing beam. In this system, optical parts such as a dichroic mirror for focusing the heating beam on a medium are necessary in addition to the heating beam light source. When the number of steps of aligning the individual light beams are taken into consideration, the apparatus is much more expensive than a common magneto-optical disk head. The size of the apparatus is also increased.
On the other hand, if reproduction is performed with a temperature gradient formed by a normal reproducing beam without using any heating beam, the position of a peak temperature is present in the reproducing beam spot as shown in FIG. 2(b). Accordingly, magnetic walls move into the reproducing spot from isothermal lines formed before and after the reproducing spot. A detailed description of this magnetic wall movement will be omitted. Details of the magnetic wall movement are described in Japanese Patent Application Laid-Open No. 6-290496.
If this is the case, as shown in FIG. 3(a) through 3(c), signals due to movements of forward and backward magnetic walls along the moving direction of the reproducing spot are detected in a region irradiated with a single beam. As shown in FIG. 3(m), the reproduction signal is a synthetic signal of signals having the same waveform, a fixed time difference, and different amplitudes.
In this state, therefore, a noise component (a signal shifted in time) is added to a necessary information signal. This makes accurate reproduction of the information impossible.